The invention relates to supercharged internal combustion engines comprising one or several valves (intake valves, exhaust valves, scavenging valves, etc.). In such engines, each poppet valve has a valve stem received in a valve guide and the end of the valve stem cooperates with actuating means (rocker, cam, etc.) shielded in a casing.
These actuating means are currently lubricated by oil splash and the sliding of the valve stem in the valve guide draws or carries along oil by a pumping effect, which ensures lubrication between the valve stem and the valve guide.
Each valve comprises a disk and a stem which is slidably received in a guide. A sufficient clearance should exist between the stem and the guide for the disk to be centered exactly with respect to its seat and for gas tightness to be achieved. A typial clearance is between 0.5 and 0.7 percent of the diameter of the stem. The actuating means are generally lubricated by oil splash and the reciprocating movement of the valve stem in the valve guide draws or carries along oil by a pumping effect which ensures lubrication between the valve stem and the valve guide. For instance, the reciprocating movement of the stem of an overhead rocker actuated valve draws lubricant from the casing into the clearance. While such a lubrication is sufficient for current engines, it is not satisfactory for engines with a high supercharging rate (particularly supercharged Diesel engines) since the pressure which prevails in the manifold passage close to the valve is quite in excess of the atmospheric pressure (higher than 6 bars in certain cases). The lubricating system should then fulfil two conditions which are contradictory to a certain extent. First, the oil pressure in that portion of the clearance which is close to the passage should be such that there is no oil "spill over" into the manifold since that would lead to an excessive oil consumption. On the other hand, the oil film between the valve guide and that portion of the valve stem which is close to the manifold should not be forced out by the gas under pressure.
In a prior art arrangement (German Pat. No. 1,236,860), a cavity in permanent communication with atmospheric pressure is formed in the inner wall of the valve guide adjacent to the valve stem and that cavity is located in the vicinity of the end of the valve guide which is closer to the valve disk. Grooves parallel to the stem axis are formed in the inner wall of the valve guide which slidably receives the valve stem and they extend from the cavity to the end of the valve guide which opens into the casing. That arrangement is not entirely satisfactory. Machining the inner wall of the valve guide is difficult. In operation, the film of oil between the inner wall of the valve guide and the valve stem may be ruptured.
In another prior art arrangement (East German Pat. No. 65,499), each valve of an engine is formed with an annular groove located close to the manifold. All annular grooves are communicated. The gas which flows along the valve stem tends to force part of the oil located in the groove of some valves (and which has flown from the casing) toward other valves. That arrangement is not satisfactory. Oil flow is pulsed rather than permanent. If the engine is supercharged, the oil flow is possible only toward the inlet valves. The volume of oil which is contained in the groove, part of which is forced into the connecting passages, is not sufficient for satisfactory lubrication of that part of the guide which is between the groove and that portion of the guide which is located close to the manifold. That oil which is forced by the pressurized gas is more or less emulsioned. Last, the flowing gas tends to rupture the oil film between the groove and the end portion of the guide.
In still another prior art arrangement (U.S Pat. No. 2,354,926), there is provided a passage for delivering pressurized oil into the valve stem clearance and an oil return passage which opens into the clearance at a point diametrically opposite from the oil inlet passage. The valve stem is formed with circumferetial grooves. Since a substantial clearance should be provided between the valve stem and the valve guide, there is a large oil leak to the intake or exhaust manifold with which the valve is associated. As an example, the oil leak along a clearance whose average value is 0.1 mm around a valve stem 16 mm in diameter is typically between 1.5 and 3.5 liters per hour, depending upon the degree of out-of-center of the stem in the guide. Such a leakage flow rate is not acceptable. In addition, the grooves formed in the valve stem are communicated with the passages only for short portions of the total duration of the cycle of operation of the engine, whereby oil renewal is not sufficiently fast.
It is an object of the invention to provide a lubrication system in which the above-mentioned short-comings are overcome.
It is a more particular object of the invention to provide a valve stem lubrication system which definitely prevents jamming, even if the valve is associated with an engine having a very high supercharging ratio and which, at the same time, does not lead to excessive oil consumption.
For that purpose, the end of the valve stem remote from the valve disk protrudes from one end of the guide into a casing which houses valve actuating means co-operable with the stem to reciprocate the valve, the valve actuating means being lubricated by oil in said chambers to which said one end of the valve guide is exposed to thereby provide a first oil supply means for feeding oil into the associated end of the radial clearance space between the stem and guide. The valve guide has first passageway means disposed circumferentially about and extending radially of said guide between the radial clearance space and the exterior of the guide to define a cavity in constant communication with the radial clearance space. The first passageway means is spaced from the other end of the valve guide by a distance measured axially of said stem which is approximately equal to the diameter of the valve stem or to the amount of reciprocating movement of the valve in operation. The valve guide has second passageway means constantly communicating said first passageway means with a pressure relief zone. Second oil supply means comprise an oil supply conduit in constant communication with the first passageway means to supply oil to the radial clearance space. The second passageway means conduct oil away from the first passageway means to the pressure relief zone.
According to another aspect of the invention, there is provided, in a supercharged internal combustion engine having at least a valve provided with a stem which is slidably received in a valve guide, the end portion of the stem remote from the valve disk co-operating with actuating means located in a casing, lubricating means comprising at least an annular cavity which permanently communicates with a return passage and is formed in the wall of the valve guide around the valve stem at a location close to the end porton of the valve guide which opens into a manifold branch associated with the valve, and a passage for feeding fresh oil into said cavity, wherein the passages are so arranged that the oil which in operation permanently flows through said cavity, communicating with the valve stem, is at a static pressure which is lower than the minimum pressure which prevails in said manifold in operation.
Due to that relation between the pressures, there cannot be any appreciable oil spill into the manifold. The end portion of the guide confronting the manifold is nevertheless lubricated satisfactorily due to the reciprocation of the valve stem which draws fresh oil from the cavity and consequently builds an oil film which prevents the gas from flowing out. The latter object is particularly fulfilled if the cavity opens in the inner surface of the valve guide at a distance from the end of the guide adjacent the manifold which is approximately equal to the amount of reciprocating movement of the valve, which is also approximately equal to the valve stem diameter.
For an appropriate static pressure to prevail in the cavity (that is a pressure which will generally be slightly higher than the atmospheric pressure, but close to the atmospheric pressure), it is sufficient that the return passage opens into the casing and that the passages be of such size with respect to each other that the head loss of the oil flow upstream of the cavity be much higher than the head loss downstream of the cavity, with the overall head loss being selected as a function of the oil inlet pressure for an optimum oil rate of flow to be achieved across the cavity. The head loss can be selected such that the pressure in the cavity of an inlet valve is different from, for instance slightly higher than, the pressure in the cavity of an exhaust valve.
Referring again to the prior art lubricating system disclosed in East German Pat. No. 65,499, it will be appreciated that there are fundamental differences in structure and operation. While in the present invention the end portion of the valve stem is lubricated with fresh oil, in said East German Patent there should inherently be a gas flow along the end portin of that valve from which emulsioned oil which has already circulated along the major portion of the valve stem is to be forced toward another valve. While the present invention may be used for lubricating an intake valve as well as an exhaust valve of a supercharged engine, there cannot be any flow of emulsioned oil toward an intake valve in the prior art device. While in the invention the permanent circulation of oil through the cavity does not interfere with the oil flow from the casing toward the cavity, which lubricates the portion of the stem remote from the valve disk, since there is no pressure build-up in the cavity, such a pressure build-up should occur in the prior art construction and apparently impedes oil flow along the upper portion of the stem. Last, it is not seen how the lower portion of intake valve stem, which is provided with the cavity from which oil is forced out to another valve, may be lubricated and apparently German patent construction relies on a bushing which cannot prevent overheating by the gas flowing from the manifold and binding.